1. Field of the Invention
The present invention relates to an optical pickup apparatus performing an operation of reading out a signal recorded in an optical disc or of recording a signal in an optical disc.
2. Description of the Related Art
There is widely used an optical disc apparatus capable of performing an operation of reading out a signal or recording a signal by irradiating a signal recording layer of an optical disc with a laser beam emitted from an optical pickup apparatus.
There is generally used an optical disc apparatus using an optical disc such as CD and DVD, while there has been recently developed an optical disc apparatus using an optical disc with improved recording density, i.e., an optical disc (new-generation optical disc) of the Blu-ray standard or the HD-DVD (High Density Digital Versatile Disc) standard.
For a laser beam for the operation of reading out a signal recorded in the optical disc of the CD standard, an infrared light with a wavelength of 780 nm is used. For a laser beam for the operation of reading out a signal recorded on the optical disc of the DVD standard, a red light with a wavelength of 650 nm is used.
There is provided a protective layer with a thickness of 1.2 mm on a top face of the signal recording layer of the optical disc of the CD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.45. There is provided a protective layer with a thickness of 0.6 mm on a top face of a signal recording layer of the optical disc of the DVD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.6.
In contrast to the above-described optical discs of the CD standard and the DVD standard, for a laser beam for the operation of reading out a signal recorded on the optical disc of the Blu-ray standard or the HD-DVD standard, a laser beam with a shorter wavelength, a blue-violet light with a wavelength of 405 nm is used, for example.
There is provided a protective layer with a thickness of 0.1 mm on a top face of a signal recording layer of the optical disc of the Blu-ray standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.85.
On the other hand, there is provided a protective layer with a thickness of 0.6 mm on a top face of a signal recording layer of the optical disc of the HD-DVD standard, and a numerical aperture of an objective lens for use in performing the operation of reading out a signal from the signal recording layer is specified at 0.65.
As described above, for the laser beam for performing the operation of reading out signals recorded on the optical discs of the Blu-ray standard and the HD-DVD standard, the blue-violet light with a wavelength of 405 nm can be used. Thus, by using a laser diode in common for the both optical discs, it may become possible to produce an optical pickup apparatus capable of performing the operation of reading out signals from the optical discs of these two standards.
In order to read out signals from the both optical discs, however, the numerical aperture needs to be changed corresponding to each of the optical discs since the location of the signal recording layer and the numerical aperture required of the objective lens are different between the two optical discs. There has been developed an optical pickup apparatus capable of performing the above-described operation (see Japanese Patent Application Laid-Open Publication No. 2006-172605).
Recently, there is commercialization of an optical disc apparatus capable of using not only optical discs of the above described CD standard and DVD standard but also optical discs of the Blu-ray standard or the HD-DVD standard, which are new-generation optical discs. Consequently, an optical pickup apparatus used for such an optical disc apparatus naturally results in being configured so as to be able to perform the operation of reading put the signals from the signal recording layer provided in the optical disc of a compatible standard or the operation of recording the signals onto the signal recording layer provided therein.
In such an optical pickup apparatus, it is difficult to apply the laser beam having the above-described wavelength onto the signal recording layer of the optical disc by a single objective lens, thereby using two objective lenses: one objective lens for applying the laser beam to the optical discs of the CD standard and the DVD standard, for example; and the other objective lens for applying the laser beam to the optical disc of the Blu-ray standard, for example (see Japanese Patent Application Laid-Open Publication Hei 11-23960).
In the optical disc apparatus capable of using a new-generation optical disc, the optical discs of the CD standard and the DVD standard, which are conventional optical discs, can also be used. The optical pickup apparatus included in such an optical disc apparatus is generally provided with two objective lenses as described above.
Such an optical pickup apparatus is configured such that either one of the objective lenses focuses laser beams different in wavelength onto the signal recording layers of the optical discs of the CD standard and the DVD standard. In order to focus the laser beams different in wavelength onto such optical discs different in standard, there is developed a technology using an objective lens with an incidence plane on which an annular diffraction grating is formed (see Japanese Patent Application Laid-Open Publication 2000-81566).
In the optical pickup apparatus, there is performed a focus control operation for precisely focusing a spot obtained by focusing the laser beam with the objective lens onto the signal recording layer of the optical disc or a tracking control operation for causing the spot to follow a signal track by utilizing three beams, i.e., a main beam that is zeroth-order diffracted light and sub beams that are ±first-order diffracted lights, which are split and generated by the diffraction grating, as is common knowledge.
In order to accommodate the optical disc of each of the standards, when the objective lens with a diffraction grating is used to diffract the laser beam of each of the wavelengths so that the laser beam is focused onto the signal recording layer, a diffraction groove of the diffraction grating is designed so as to improve a diffraction efficiency of the diffracted light obtained from the laser beam of each of the wavelengths. In designing such a diffraction grating, it is difficult to secure the optimum diffraction efficiency for the laser beams of all the wavelengths, and therefore, the diffraction grating is designed with sacrificing diffraction efficiency for a diffraction grating of a laser beam of a wavelength low in priority.
For example, since the maximum light output is lower in the laser beam with shorter wavelength in the present state, the priority of the diffraction efficiency needs to be given to an optical disc having a higher recording density, and thereby sacrificing a diffraction efficiency for a laser beam for CD of which the generation is the oldest and the recording density is the lowest. In this case, although it depends on the diffraction efficiencies set for the laser beams for other optical discs, in some designs, the diffraction efficiency in the laser beam for CD becomes about 40%, which is a value lower than that of an unnecessary diffracted light (about 60%) called a flare light.
With the diffraction efficiency like this, there increases an amount of flare light occurring from the main beam whose light intensity is the highest among the above-described three beams, which is received by a light receiving region for a sub beam provided in a photodetector, to an unignorable degree, which leads to a problem that the tracking servo operation or the focus servo operation becomes unstable.
The diffraction grating formed on the objective lens is designed such that the numerical aperture is appropriate for each of the optical discs according to the wavelength of the laser beam for CD and that of the laser beam for DVD. The optical pickup apparatus has characteristics that when the numerical aperture of the objective lens is reduced, degradation of characteristics based on an inclination of the objective lens relative to a signal surface of the optical disc is reduced.
In an optical pickup apparatus configured such that an operation of focusing the laser beam onto the signal recording layer provided in the optical disc of the CD standard and an operation of focusing the laser beam onto the signal recording layer provided in the optical disc of the DVD standard are performed with the objective lens formed with the annular diffraction grating, the inclination of the objective lens or the like is set with giving higher priority to characteristics of the optical disc of the DVD standard, and therefore, characteristics for the optical disc of the CD standard is degraded.
In order to improve these points, there may be considered a method of reducing the numerical aperture of the objective lens which is set correspond to the optical disc of the CD standard with taking the above-described characteristics into consideration. When the numerical aperture is reduced, however, there increases the amount of flare light received by the light receiving region provided in the photodetector to an unignorable degree. As a result, it becomes difficult to obtain an exact focus error signal, which leads to a problem that the focus servo operation performed with the use of the focus error signal becomes unstable.